Chatham House Rule greeter

Supplybot should greet those who join a channel operating under the Chatham House Rule with a message reminding them and others that the channel is under that rule.

The text should contain:

Participants in this channel agree to observe the 
<https://en.wikipedia.org/wiki/Chatham_House_Rule|Chatham House Rule>. 
Participants are free to use the information received, 
but neither the identity nor the affiliation of the speaker(s), 
nor that of any other participant, may be revealed.

The channel list should be the combination of two things: a list in the source code – for public rooms – combined with the content of an environment variable – for private rooms.

Work with @justinxreese and @colindean to finalize any additional language.